1. Field of the Invention
The present invention relates generally to volume holograms and more specifically to volume holograms for use in optical data storage systems.
2. Description of the Prior Art
Magneto-optic recording allows the erasable storage of data. A write laser beam is focussed onto a spot on the medium and heats the magneto-optic material to a temperature at which the magnetization of the magnetic domains of the medium may be changed. This is known as the Curie temperature. A magnetic field is applied in one of two directions to orient the magnetic domain of the spot in either an upward or downward direction.
The disk is read by focussing a polarized read laser beam onto the magneto-optic material. The read laser beam has a lower power level than the write laser beam and does not heat the medium to the Curie temperature. The Kerr effect causes the plane of polarization of the beam of light reflected from the medium to be rotated either clockwise or counter-clockwise depending on whether the spot has an upward or downward magnetic orientation. The difference in rotation is then detected and represents the recorded data.
The typical system uses one laser with variable power to do both writing and reading. In order to verify the data recorded, the disk must be rotated almost three times for each track recorded. One and a half revolutions are required on average to seek and write the track and a second revolution is required to read and verify the track just written.
In order to speed up the recording process, direct read after write (DRAW) systems have been proposed. These systems comprise two lasers; one laser (the read/write laser) to write a track and a second laser (the DRAW laser) to read the track directly after it has been written. Thus, the DRAW system requires only one and one half revolutions on average to write and verify a track on the disk. The read/write laser alone is used to read the disk when no writing is being performed.
Another type of optical data storage system uses multiple beams to read and record several parallel tracks at once. The beams are focussed onto separate tracks of the optical disk. The optical channel receives a reflected beam from each of the tracks and generates a data signal responsive thereto. Data may be recorded and read much faster in this parallel manner.
A problem with both the DRAW and parallel track systems is that the individual beams must be closely spaced in order to use the same optical channel. Typically it is desired that the two beams be offset from one another at an angle of five degrees or less. The beams are generated by separate laser sources and it may not be possible to position the laser sources close enough together in order to achieve the small spacing of the beams. What is needed is a system which combines two widely divergent beams from separate laser sources into two closely spaced beams which have a small angle of deviation.